The invention relates, in a general sense, to assessing the susceptibility of mammals to carcinogenesis.
An individual's susceptibility to cancer is governed by the individual's genome and carcinogenic stimuli which the individual encounters in the environment. Although the carcinogenic potentials of many compounds and other stimuli (e.g. ionizing radiation) have been determined, assessing the importance of limiting exposure to such compounds is complicated by the fact that not all individuals are equally susceptible to the carcinogenic effects of the compounds. Thus, the genetic component of carcinogenic susceptibility limits accurate prediction of cancer rates among individuals, even in defined environments.
Prior art observations suggest that up-regulation of the mammalian gene (odc) encoding ornithine decarboxylase is associated with enhanced susceptibility to carcinogenesis (Luk et al., 1984, N. Engl. J. Med. 311:80-83). Other prior art references indicate that overexpression of the odc gene increases susceptibility of mammalian cells for the tumor promotion stage of carcinogenesis, but that over-expression of odc is not, by itself, a sufficient condition for carcinogenesis (Clifford et al., 1995, Cancer Res. 55:1680-1686; Auvinen et al., 1992, Nature 360:355-358; Moshier et al., 1993, Cancer Res. 53:2618-2622; Hibshoosh et al., 1991, Oncogene 6:739-743; O'Brien et al, 1997, Cancer Res. 57:2630-2637).
It is known that there are at least two alleles of the human odc gene, which have been observed as different PstI restriction fragment length polymorphisms (RFLP; Hickok et al., 1987, DNA 6:179-187).
It is believed that the odc gene product, ornithine decarboxylase, is involved in establishing cellular polyamine levels, and that the susceptibility of a tissue to carcinogenesis is related to polyamine levels in the cells of the tissue. Transcription of the odc gene is activated by the protein designated Myc (Bello-Fernandez et al., 1993, Proc. Natl. Acad. Sci. USA 90:7804-7808; Tobias et al., 1995, Oncogene 11:1721-1727; Wagner et al., 1993, Cell Growth Diff. 4:789-883). Such regulation is effected by binding of Myc/Max heterodimers to one or more of the three Myc binding elements (also called "E-boxes") in the human odc gene. These E-boxes are located at nucleotide residues -485 to -480 (relative to the transcription start site) in the odc 5'-promoter region and at nucleotide residues +292 to +297 and +326 to +331 in the first intron of the odc gene. The fact that the E-box located in the promoter region of the human gene is not conserved in the rat and mouse genes, suggests that the other two E-boxes are the physiologically important regulatory elements.
In normal cells, expression of c-myc (the gene encoding Myc protein) is associated with cell cycle progression. Lack of c-myc expression permits cells to withdraw from the cell cycle and cease differentiating. Furthermore, constitutive c-myc expression promotes continuous cell cycle progression independent of growth factors (Rapp et al., 1985, Nature 317:434-438). Over-expression of c-myc is a common characteristic of many (if not most) human tumors. Prior art studies have correlated c-myc expression and odc expression in human tumors, but not in surrounding normal (i.e. non-tumor) tissue (Gan et al., 1993, J. Histochem. Cytochem. 41:1185-1196; Mori et al., 1996, Cancer 77{8 Suppl.}:1634:1638; Mimori et al., 1997, Dis. Colon Rectum 40:1095-1100). These studies suggest that deregulation of c-myc expression may lead to deregulation of odc expression.
Numerous stimuli are known which induce deregulation of c-myc expression (Schwab et al., 1984, Proc. Natl. Acad. Sci. USA 81:4940-4944; Payne et al., 1982, Nature 295:209-214; Klein et al., 1985 Immunol. Today 6:208-215). Because deregulated Myc transactivates odc expression, these stimuli can be predicted to induce deregulation of odc expression in general. However, until the present invention, it has not been known whether either of the alleles of odc is more susceptible to transactivation by Myc, and thus the genetic component of carcinogenic susceptibility of an individual could not be ascertained.